Fatigue reistant foundation system

ABSTRACT

A fatigue resistant gravity based spread footing under heavy multi-axial cyclical loading of a wind tower having a central vertical pedestal, a substantially horizontal continuous bottom support slab with a stiffened perimeter, a plurality of radial reinforcing ribs extending radially outwardly from the pedestal and a three-dimensional network  500  of post-tensioning elements that keep the structural elements under heavy multi-axial post compression with a specific eccentricity that is intended to reduces stress amplitudes and deflections and allows the foundation to have a desirable combination of high stiffness and superior fatigue resistance. The foundation design reduces the weight and volume of materials used, reduces cost, and improves heat dissipation conditions during construction by having a small ratio of concrete mass to surface area thus eliminating the risk of thermal cracking due to heat of hydration.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for building fatigue resistantfoundations for supporting columns, towers and structures under heavycyclical loads such as wind turbines towers for onshore and offshoreinstallations.

The proposed foundation system will be used specifically for theMulti-Megawatt class wind turbines. Several wind turbine manufacturershave successfully developed large wind turbines with rated power rangingfrom 1.5 to 10 MW. Several wind turbine manufacturers are planning massproduction of large multi Megawatt turbines for onshore and offshoreinstallations. The installation of the E-126 model turbine by Enerconwith a 7 MW rated power required a xx meter diameter circular foundationwith 1,400 cubic meters of concrete and xx tons of rebar. Theinstallation of the 5M turbine by RePower with a 5 MW rated powerrequired a 23 meter diameter circular foundation with 1,300 cubic metersof concrete and 180 tons of rebar. The task of building such largefoundations is monumental and requires a great deal of constructionplanning and logistics. The proposed foundation designs and theirassociated construction methods provide cost-effective solutions forsuch challenging foundation projects.

Several wind turbine foundations, that have been constructed in the last10 years in the US and Europe, have structural problems stemming fromthermal cracking during construction or from fatigue cracking andrequired repairs. The present invention improves the geometry of thefoundation in order to enhance dissipation conditions for the due to thetypical temperature rise after casting and also provides a costeffective fatigue resistant design.

2. Description of the Related Art

Conventional gravity style foundations for large wind turbine usuallycomprise a large, thick, horizontal, heavily reinforced cast in situconcrete base; and a vertical cast in situ cylindrical pedestal that isinstalled over the base. There are several problems that are typicallyencountered during the construction of such foundations.

Fatigue resistant of such conventional footings is achieved by oversizing the structural concrete elements and the reinforcing elementssuch that the resulting stress amplitudes are small enough for thestructural elements to bass fatigue design checks.

The main problem is the monumental task of managing large continuousconcrete pours, which require sophisticated planning and coordination inorder to pour more than four hundred cubic yards of concrete perfooting, on average, in one continuous pour, without having any coldjoints within the pour.

Another problem is logistics coordinating with multiple local batchplants the delivery plan of the large number of concrete trucks to thejob site in a timely and organized manner.

A further problem is the complexity of installing the rebar assemblyinto the foundation which requires assembling two layers of steelreinforcing meshes that are two to six feet apart across the full areaof the foundation, while maintaining strict geometric layout andspecific spacing. This rebar assembly is made of extremely long andheavy rebar which requires the use of a crane in addition to multipleworkers to install all components of the assembly. The rebar oftenexceeds forty feet in length, thus requiring special oversized shipmentswhich are very expensive and usually require special permits. That laborintensive and time consuming task requires large number of well trainedrebar placing workers.

Another important problem is the fact that majority of the constructionprocess consist of field work which could be easily compromised byweather conditions and other site conditions.

Another problem is thermal cracking of concrete due to overheating ofthe concrete mass. When concrete is cast in massive sections for windtower foundations, temperature can reach high levels and the risk ofthermal cracking becomes very likely. Thermal cracking often compromisesthe structural integrity of the foundations as reported in many projectsin Europe and North America.

Multi-cell caissons used in offshore installations always lackmulti-axial post-tensioning elements and their fatigue resistant relycompletely on heavily reinforced oversized concrete elements whichinvolves expensive and labor intensive construction.

BRIEF SUMMARY OF THE INVENTION

It is desired to have cost-effective foundation system that can reduceconstruction materials and labor for large wind turbines. The windturbine foundations can then be built to the standards of the FatigueResistant Foundation System which uses concrete rib stiffeners, with acast in place slab on grade element and a central pedestal to build anintegral foundation that will behave structurally as a monolithicfoundation structure. Other concrete components can be included such assecondary and perimeter beams, diaphragms, or intermediate stiffenersand rib stiffened or flat slab sections. The foundation system relies onthe use of many prefabricated components including rebar meshes andcages, pedestal cage assembly, pre cut post-tensioning strands,preassembled strand bundles, pre-cut post-tensioning duct sections andprefabricated concrete forms.

The present invention pertains to a fatigue resistant foundation forwind towers which comprises a plurality of components, namely a centralvertical pedestal, a substantially horizontal continuous bottom supportslab with stiffened perimeter, a plurality of radial reinforcing ribsextending radially outwardly from the pedestal and a three-dimensionalnetwork of vertical, horizontal, diagonal, radial and circumferentialpost-tensioning elements embedded in the footing that keeps all thestructural elements under heavy multi-axial post compression, reducesstress amplitudes and deflections and allows the foundation to have adesirable combination of high stiffness and superior fatigue resistantwhile improving heat dissipation conditions during construction byhaving a small ratio of concrete mass to surface area thus eliminatingthe risk of thermal cracking due to heat of hydration.

Although the application is written for a wind turbine tower as thecolumn being supported by the foundation, any tower or column can beused on the foundation including but not limited to, antennas, chimneys,stacks, distillation columns, water towers, electric power lines,bridges, buildings, or any other structure using a column.

In one embodiment, the present invention pertains to a wind turbinefoundation having a plurality of components, namely a central verticalpedestal, a substantially horizontal bottom support slab, and aplurality of radial reinforcing ribs extending radially outwardly fromthe pedestal. The ribs are prefabricated and transported to job site,but the pedestal and support slab are poured in situ at the site out ofconcrete. The prefabricated ribs are equipped with load transfermechanisms, for shear force and bending moment, along the conjunctionswith the cast in situ support slab. The prefabricated ribs are alsoequipped at their inner ends with load transfer mechanisms, for shearforce and bending moment, along the conjunctions with the cast in situpedestal. The ribs are arranged in a circumferentially spaced manneraround the outer diameter of the pedestal cage assembly before or afterslab reinforcing steel is installed. Forms are then arranged for thepedestal and support slab. The support slab is cast in situ by pouringconcrete into the forms and then pedestal concrete is poured over theslab into the pedestal form. When the concrete cures the support slab isunited to the prefabricated ribs and the ribs are also united to thepedestal. The final result is continuous monolithic polygonal orcircular foundation wherein loads are carried across the structurevertically and laterally through the continuous structure by the doweledand spliced reinforcing steel bars which are integrally cast into thepedestal, ribs and support slab. The combination of the high stiffnessof the ribs, solid pedestal and continuous slab construction across thepedestal, and through or under ribs, allows the slab to behavestructurally as a continuous slab over multiple rigid supports resultingin small bending and shear stresses in the slab, reducing deflectionsand increasing the stiffness of the foundation, improving fatigueconditions as well as allowing for the benefits of an economical design.Support slab reinforcing steel covers the entire footprint of thefoundation and extends, without interruption, across the slab area andinto the pedestal to improve the structural performance of thefoundation under different loading conditions. Perimeter beams orthickened slab edges around the perimeter add stiffness and strength tothe foundation and provide the benefits of a two-way slab system.Circumferential post-tensioning of slab edge is used to increase thestructural capacity of the ribs and the pedestal by creating eccentricpost compression in the ribs and by reducing stress amplitudes in theslab, ribs and pedestal.

The foundation of the present invention substantially reduces the amountof concrete used in wind turbine foundation of spread footing style,simplifies the placement of rebar and concrete in the foundation, allowsfor labor and time savings and shortens foundation construction schedulewhen compared to conventional designs.

This invention provides the wind energy industry with a foundationsystem suitable for utility scale wind turbines including 1.5 MW through10 MW and even larger, wherein the amount of cast in situ concrete workis limited, and the number of concrete trucks required for thefoundation is small and manageable level and the amount of rebar used inthe foundation is 60% less than conventional footings.

In one embodiment, the present invention relies on using prefabricatedcomponents that meet size and weight limits for standard ground freightshipping through typical roads and highways, without resorting tospecial permitting for oversize or overweight shipments, keeping in mindthat the foundation width for large turbines can easily exceed sixtyfeet in width.

In another embodiment, the present invention uses specific combinationsof precast components with cast in situ components designed to speed upconstruction without compromising the rigidity and structural continuityand optimization of the foundation. The combination of high strength,high stiffness prefabricated ribs, solid pedestal construction andcontinuous slab construction across the pedestal, and through or underribs, allows the slab to behave structurally as a continuous slab overmultiple rigid supports resulting in small bending and shear stresses inthe slab, reducing deflections and increasing the stiffness of thefoundation, substantially reducing fatigue as well as allowing for thebenefits of rapid construction and economical design.

The present invention improves the geometry of the foundation in orderto enhance dissipation conditions for the heat of hydration due to thetypical temperature rise after casting. This design feature is achievedby reducing the thickness of the support slab and the ratio of concretemass to surface area, thus reducing the risk of thermal cracking andprotecting the structural integrity of the foundations.

The present invention optimizes the design support slab by configuringslab reinforcing to span between supporting ribs and allowing it tocontinue under or across the ribs. Each slab panel is triangular orpie-shaped and is prestressed along all three sides such that amulti-axial prestress is generated in slab panel. Slab panels withradial and perimeter post tensioning elements form a robust horizontaltrussed diaphragm and as a result, the required slab thickness isoptimized and the amount of cast in situ concrete is reduced.

The present invention reduces the maximum rebar length for fieldinstallation to roughly 7.6 meters (twenty five feet), which issignificantly shorter when compared to conventional footing that mayrequires 15.2 to 18.3 meters (fifty to sixty foot) long reinforcingbars.

The present invention allows rib dowels, or post tensioning tendons,extending inwardly into the pedestal at one end, to continue withoutinterruption between distal ends of the foundation. As a result eachpair of ribs on opposite ends of the pedestal will behave structurallyas one continuous beam across the width of the foundation.

The present invention reduces fatigue for concrete and rebar in thefoundation by minimizing stress concentrations through appropriatelyconfigured connections and component geometry. The solid and deepconstruction of the pedestal allows for great reduction of stressesacross the pedestal and at the conjunctions between the pedestal andsurrounding. Dowels into the pedestal are relatively deep to reducestresses near the surface zone of the pedestal. The solid pedestaloffers generous bearing conditions for the tower base plate and improvesgeometry as needed to minimize fatigue.

The present invention employs prestressing and/or post tensioningtechniques in order to maximize the performance of the foundation, or toextend its life span. Besides the vertical tensioning of anchor bolts,tensioning of horizontal and diagonal tendons along the length concreteribs and across the pedestal besides circumferential 112 and radial 111post tensioning strands imbedded in the slab are employed.Post-tensioning of the ribs is done in an eccentric manner to counterbalance and reduce the stresses from the dead loads on the foundation.This can be accomplished by setting an eccentric post tensioning loadpattern in the ribs with higher axial force at the bottom than at thetop of the rib. The circumferential post tensioning load in the slabprovides additional desirable eccentric prestressing of the ribs and thepedestal and helps increase rib load capacity and rib fatigueresistance.

OBJECTS OF THE INVENTION

An object of this invention is to provide the wind energy industry witha fast, reliable, yet cost-effective foundation system that is suitablefor most wind energy projects, including projects using the largestutility scale turbines and tallest towers, while providing a foundationlifespan that is longer than conventional foundation systems.

Another object of this invention is to reduce the cost of wind energyprojects by realizing savings in the areas of rebar quantity, form work,concrete trucking service, concrete pouring and finishing, logistics,man-hours and crane operations.

It is the object of this invention is to provide foundation systemsuitable for large wind turbines including utility scale turbinesranging from 1.5 MW to 10 MW and larger, wherein the amount of cast insitu concrete work is limited and the number of concrete trucks and theamount of rebar required for the foundation are reduced to a manageablelevel when compared to conventional gravity style foundations.

Another object of this invention is to improve dissipation conditionsfor the heat of hydration and the typical temperature rise aftercasting. That goal is achieved by reducing the ratio of concrete mass tosurface area. When concrete is cast in massive sections for wind towerfoundations, temperature can reach high levels and the risk of thermalcracking becomes very high unless cooling techniques or specialadmixtures are applied. Thermal cracking often compromises thestructural integrity of the foundations.

A further object of this invention is to improve foundation structuralproperties due to fabrication of some structural components in a fullycontrolled environment of a precast concrete plant or a suitablefacility at or near project site and to utilize benefit from advancementin concrete construction in areas such as concrete admixtures, specialcements and fiber reinforcement.

Still another object of this invention is to utilize desirable featuresand benefits associated with mass production of precast concrete such ashigh reliability and uniform consistency and high compressive strength.

Another important object of this invention is to minimize chances forerrors in bar placement, spacing and layout by providing pre-markedspacing for splicing slab rebar with existing dowels extending fromribs.

A further object of this invention is to use light weight, smalldiameter, short and easy to handle rebar for the cast in situ concrete.

A further important object of this invention is to provide the windenergy industry with a solution for all weather foundation construction.

Still another important object of this invention is to improve safetyand accessibility around foundations under construction, and reducehazardous conditions for construction crew.

A further significant object of this invention is to increaseproductivity and increase the number of footing that can be built in agiven time frame using the same number of workers, when compared toconventional foundation designs built under similar conditions.

Another object of this invention is to employ prestressing and/or posttensioning techniques in order to maximize the performance of thefoundation, improve its fatigue resistant and extend its life span.

Another object of this invention is to provide the wind energy industrywith reliable and readily available designs, and prefabricatedcomponents, for every wind energy project wherein foundation designs arepre-approved by and coordinated with turbine manufactures andcertification agencies.

A further object of this invention is to use standard designs to reduceengineering work and simplify the permitting process, as well as improveproject construction schedule.

Still another object of this invention is to speed-up construction byusing many prefabricated components including rebar meshes and cages,bolt cage assembly, pre cut post-tensioning strands, preassembledpost-tensioning bundles, pre-cut post-tensioning duct sections andprefabricated concrete forms.

It is also the object of this invention is to provide wind energydevelopers with the ability to select pre-approved complete foundationdesigns for wind turbine foundation based on project and site variablesincluding turbine model and tower height; site geotechnicalcharacteristics; and desired foundation style such as gravity, anchoredor piling.

Another object of this invention is to provide foundation contractorswith the convenience and economy of using commercially availableprefabricated components with complete assembly and detail drawings thatcan be delivered to any project site with short lead time.

A further object of this invention is to improve the quality andproductivity of foundation construction due to experience gained frompracticing standard construction techniques with repetitive productionsteps.

Still another object of his invention is to produce foundation designssuitable for shallow and deep offshore installations.

Another object of this invention is to use the modular foundation systemfor other tower structures such as chimneys, stacks, distillationcolumns and telecommunication towers.

Yet another object of the foundation is to improve tower base bearingresistant in concrete pedestals supporting wind towers such that itbecome possible to build the pedestal and the foundation with concretehaving the same compressive strength without increasing the diameter ofthe pedestal.

Another object of the invention is to build wind tower foundation in onecontinuous concrete pour.

The final object of the invention is to independently produceprefabricated components for offshore foundations that can be assembledon a barge without having the critical path of completing to a firstcomponent before a second component can be constructed.

Other objects, advantages and novel features of the present inventionwill become apparent from the following description of the preferredembodiments when considered in conjunction with the accompanyingdrawings.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the foundation showing the rebar beforepouring the concrete.

FIG. 2A is a perspective view of a pedestal and ribs in a secondembodiment with a pier for off shore applications.

FIG. 2B is a perspective view of a pedestal and ribs.

FIG. 3A is an inner perspective view of a rib showing connections to thepedestal and the slab.

FIG. 3B is an outer perspective view of a rib showing connections to thepedestal and the slab.

FIG. 4 is a perspective view of a rib and forms for forming the pedestaland slab.

FIG. 5 is a perspective view of the bolt assembly and alignmentapparatus.

FIG. 6 is a top view of the foundation prior to pouring the concreteshowing the rebar and template for the anchor bolts and post tensioningelements.

FIG. 7 is a perspective view of a raised rib having means for raisingthe rib above the slab.

FIG. 8 is a perspective view of the foundation showing the alignmentapparatus and a pedestal forming section.

FIG. 9 is a perspective view of the foundation showing the rebar andrebar cage.

FIG. 10 is a perspective view pedestal cage assembly with anchor boltand reinforcing.

FIG. 11 is a perspective view of the foundation.

FIG. 12 is a perspective view of the rib for supporting a lattice styletower.

FIG. 13 is a perspective view of the foundation for offshoreinstallation.

FIG. 14 is a perspective view of the foundation for offshoreinstallation.

FIG. 15 is a perspective view of the foundation for offshoreinstallation.

FIG. 16 is an elevation view of the foundation for offshoreinstallation.

FIG. 17 is a perspective view of the foundation for offshoreinstallation.

FIG. 18 is a perspective view of the foundation for offshoreinstallation.

FIG. 19 is a perspective view of the foundation for offshoreinstallation.

FIG. 20 is a perspective view of the foundation with rock anchors.

FIG. 21 is a perspective view of the foundation with rock anchors.

FIG. 22 is a perspective view of the foundation with rock anchors.

FIG. 23 is an elevation view of the foundation with rock anchors.

FIG. 24 is a perspective view of an offshore foundation withmicro-piles.

FIG. 25 is an elevation view of an offshore foundation with micro-piles.

FIG. 26 is a perspective view of the foundation.

FIG. 27 is an elevation view of the foundation.

FIG. 28 is a perspective view of rock anchored foundation.

FIG. 29 is a perspective view of rock anchored foundation.

FIG. 30 a-30 d show circumferential post tensioning view of thefoundation.

FIG. 31 a is a plan view of the foundation.

FIG. 31 b is a section view of the foundation.

FIG. 32 a is a section view of the foundation.

FIG. 32 b is a section view of the rib 16.

FIG. 33 a is an elevation of rib reinforcing details.

FIG. 33 b is a plan view of rib reinforcing details.

FIG. 34 a and FIG. 34 b are section views of the rib 16.

FIG. 34 c and FIG. 34 d are pedestal reinforcing details.

FIG. 35 a and FIG. 35 b are section views of the pedestal.

FIGS. 36 a and 36 b are slab 20 reinforcing plans.

FIG. 37 a and FIG. 37 b are elevation and plan views of a rib withunbounded post tensioning elements.

FIG. 38 FIG. is a plan view showing circumferential post tensioning inthe foundation.

FIG. 39 a-FIG. 41 b show details of the a foundation with prefabricatedribs.

FIG. 42 a-FIG. 42 d show tendon duct arrangements in a pedestal 10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention pertains to a wind turbine foundation for windturbines. The foundation comprises a plurality of components, namely acentral vertical pedestal, a substantially horizontal bottom supportslab, and a plurality of radial reinforcing ribs extending radiallyoutwardly from the pedestal. The ribs may be prefabricated andtransported to job site, but the pedestal and support slab are poured insitu at the site out of concrete. Alternatively the ribs may be cast insitu.

The present invention pertains to a fatigue resistant foundation 100 forwind towers which comprises a plurality of components, namely a centralvertical pedestal, a substantially horizontal continuous bottom supportslab with a stiffened perimeter, a plurality of radial reinforcing ribsextending radially outwardly from the pedestal and a three-dimensionalnetwork 500 of vertical, horizontal, diagonal, radial andcircumferential post-tensioning elements embedded in the footing thatkeeps all the structural elements under heavy multi-axial postcompression, reduces stress amplitudes and deflections and allows thefoundation 100 to have a desirable combination of high stiffness andsuperior fatigue resistance while improving heat dissipation conditionsduring construction by having a small ratio of concrete mass to surfacearea thus eliminating the risk of thermal cracking due to the heat ofhydration.

A construction site is prepared by excavation and flattening andpreparation of soil for the foundation 100. The foundation 100 may beset on pilings, on piers, or have anchors (soil anchors or rock anchors404 or micro-piles 401 or other types) in a conventional manner. Thepresent invention ensures good contact between foundation 100 and soil,or sub-base, by casting.

The foundation 100 is cast against prepared soil, or crushed stonesub-base, or a mud slab or a membrane sheet in case of offshorefoundations assembled on a barge or in dry docks. Known grouting andleveling techniques under precast elements can be employed for ensuringplumb installation and good soil contact.

In one embodiment of the invention the foundation 100 may be set on amud slab 14 or on compacted granular fill. The mud slab 14 is often athin plain concrete layer intended to provide a clean and level base forfoundation installation. After the foundation site has been prepared, aplurality of three or more precast stiffener ribs 16 are placed on themud slab 14 or compacted granular fill inside of the excavation pit 12.The precast concrete stiffener ribs 16 may have means for leveling orother leveling techniques can be employed for level and plumbinstallation. If desired, grouting techniques can be used to ensurecomplete rib base contact with the mud slab or sub-base. The precastconcrete stiffener ribs 16 may have bases 21 with left shear key 38and/or shear connectors and right shear key 36 and/or shear connectors.The precast concrete stiffener ribs 16 may also have a vertical shearkey 34. The shear keys 34, 36 and 38 and associated dowels 40, 42 and 46are to ensure continuous connections, with complete transfer of shearand bending loads, between the precast concrete rib stiffener 16 and thecast in place concrete which is to be poured into the foundation 100.The precast concrete stiffener ribs 16 have upper dowels 40 and lowerdowels 42 extending on the right and left sides of the base 21 whichinterconnect with and spliced to upper mesh rebar 22 and lower meshrebar 24 installed between the ribs 16 and connected to dowels 40, 42 toform reinforcement for the slabs of foundation 100 when the concrete ispoured. The base 21 of rib 16 and the top of rib 16 also have dowels 46radially entering the pedestal 10 in the center of the foundation.

Doweling of rebar between ribs and foundation components can be achievedrebar dowels extending from the prefabricated elements or by using rebarcouplers, bar extenders or any mechanical rebar splicing system.

Arrays of grout or epoxy filled sleeves arranged in the slab 20 couldreceive corresponding arrays of vertical dowels extending from thebottom of prefabricated ribs or perimeter beams 190 or otherprefabricated components.

Shear keys can be replaced with, or combined with, corbels or shearstuds, or other shear connectors such as angled rebar or embedded steelshapes.

In another embodiment an array of steel beams, are encased into the webof the rib and extend inwardly into the pedestal cavity at the innermost end of ribs, and serve as a suitable shear force transfer mechanismbetween the rib and the pedestal.

In another embodiment the foundation 100 comprises a steel frame fullyis encased in concrete and has a central tower receiving metal cylinderfixed to an array of radially extending steel girders encased inconcrete beams and rigidly connected at their outer ends to an array ofperimeter beams 190 encased in the concrete foundation and a reinforcedconcrete slab-on-grade 20 covering the foot print of the foundation 100and connected to the said steel frame.

In one embodiment the ribs are treated with concrete bonding agent alongsurfaces where cast in place concrete is received.

In another embodiment the foundation 100 is provided with drains aroundthe perimeter and the top surface of the slab 20 is slightly slopedtowards the said drains such that water is drained away from foundation100.

In another embodiment the ribs or other foundation elements are coveredor coated with protective material for extending the life span of thefooting.

In one embodiment the ribs 16 are placed on the mud slab 14 first andthen the pedestal cage 50 made of an array of rebar, preferably Z or Cshaped rebar and circumferential rebar is assembled around anchor boltassembly. Alternatively the pedestal cage 50 is assembled first or apreassembled pedestal cage 50 dropped into place first and then the ribs16 with dowels 46 are slid into place so that dowels 46 and shearconnectors fit between the elements of pedestal cage 50 rebar assembly.

As best seen in FIG. 3 b, the precast concrete stiffener rib 16 haslifting lugs 32 to help place the stiffener rib 16 into the excavatedconstruction area. The base 21 has a flat bottom surface such that theribs may stand on their own on the mud slab 14 or compacted granularfill or during transportation from precast plant to foundation site. Theprecast concrete stiffener ribs 16 have prestressing elements 58 runningthrough the ribs 16 radially from the outside of the ribs 16 and throughpedestal 10. The radial prestressing elements 58 (or post tensioningelements) may be anchored to the opposite side of the pedestal oroptionally run through the opposing precast concrete stiffener 16 on theother side of the pedestal 16 and anchored at the end of the oppositerib 16. Once the ribs 16 and the pedestal cage 50 are in place, thedowels 46 extending radially inward from ribs 16 may be connected to, orspliced with, corresponding dowels arranged in the pedestal cage. Insideof a cage 50 are additional rebar dowels 48 which will facilitate thecontinuity of the structural components through the pedestal 10 as wellas resist bearing, shear and bending loads.

Also inside of pedestal reinforcement cage 50 is a bolt assembly 60comprising a bolt template 52 an embedment ring 54 and anchor bolts 56protected by a PVC sleeve 57 or wrapped with a material to preventbonding between the anchor bolts 56 and concrete to be poured. Theanchor bolts 56 have a top portion which is used to attach the baseflange 301 of a tower or column to the pedestal 10. A grout troughtemplate 52 at the bottom of the bolt template 52 may be used to createa grout trough 90 to ensure a good connection of the tower or column tothe pedestal 10. The grout trough 90 will be formed by removing the bolttemplate 52 from the anchor bolts 56 after the concrete has been poured.Radial dowels, prestressing elements or shear connectors at the innerend of ribs 16 should be spaced to clear anchor bolts # and otherreinforcement arranged in pedestal cage 50.

In a preferred embodiment, for fully cast in place foundations, slabforms may sit directly on the mud slab and rib forms 16 b are supportedand kept elevated above slab 20 elevation by means of adjustable andreusable support legs arranged in the rib forms 16 b. Small footings orthickened mud slab areas could be used under rib form support legs.Pedestal forms 102 can be supported by rib forms 16 b or by separatesupport legs.

When ribs 16 are prefabricated, the bolt assembly 60 is held in placeand the anchor bolts 56 are properly oriented by an alignment apparatus130 can be utilized. The alignment apparatus 130 has a central post 132with arms 134 attached perpendicularly to the center post and havinglegs 136 for attachment to the top of the ribs 16 to provide addedstability, and bolt circle proper alignment during construction. Thelegs 136 have an adjustable height relative to the arms 134.

The arms 134 may have braces 138 attached to the central post 132 forholding the arms 134 straight. The central post 132 may also have rodsupports 135 for holding reinforcement rebar such as reinforcement rebar80 which are spliced to dowels 46. The alignment apparatus 130 also hasadjustable support members 140 for attachment between the arms 134 andthe bolt template 52 to align the anchor bolts 56 so they are upright.The alignment apparatus 130 can support the bolt assembly 60 without thecentral post 132 by relying on the legs 136 supported by ribs 16, whichallows the lower portion of the central post 132 to be removed ifdesired. Alignment apparatus 130 can be used as a template to ensureproper location, elevation and orientation of ribs 16.

The ribs 16 can be of any shape or size depending on the specificationsof the tower and loads thereon. For example the ribs 16 may betrapezoidal, rectangular, tee shaped or I beam shaped. The ribs 16 mayhave intermediate stiffener plates or diaphragms for improved structuralperformance. The ribs 16 or rib forms 16 b may receive ramps or catwalksthereon for easy access to the forms during construction.

Ribs 16, or rib forms 16 b, may have means for receiving and supportingperimeter forms 18, such as bolts or threaded inserts for receiving andsupporting the pedestal forms 102. The ribs 16, or rib forms 16 b, mayalso have attachment means 15 for holding base forms 17. The pedestalforms 102 may be equipped with platform sections for allowing accessaround the pedestal and the rest of the footing.

With all the rebar, ribs 16, pedestal 100, bolt assembly frame 80 andoptional alignment apparatus 130 in place concrete forms may be attachedsuch that concrete can be poured to form the pedestal and base of thefoundation. Pedestal forms 102 may attach to the ribs 16, or rib forms16 b, by bolts 18 or by any other means. Similarly the base perimeterforms 17 may be attached to the ribs 16, or rib forms 16 b, by bolts 15or by any other means. Alternatively the base perimeter forms may besupported to the ground or the mud slab.

With all the parts assembled all the rebar in place and the conduit forthe prestressing tendons or rods of the foundation in place, concrete isready to be poured into the pedestal 10 and between the ribs 16. Thepouring of the concrete can be accomplished quickly and slab areasbetween the ribs 16 can be finished as the pedestal 10 concrete is stillbeing poured. The concrete may be used to build the pedestal 10 and theslab 20 in one pour. Alternatively the base for the entire foot print ofthe footing can be poured in a first pour then the pedestal 10 can beformed in a second pour.

When bonded multi-strand post tensioning system is used in thefoundation 100, the prefabricated components will be fitted with ductsand anchor hardware according to design specifications. The cast inplace components will be fitted with matching ducts to facilitate thecontinuity of tendons across the foundation 100. After the jacking oftendons, duct grouting is carried out as required. If the un-bonded,bundled mono-strand system is employed, no duct or grouting is required.

The structural load capacity of the foundation 100 is increasedsignificantly by the combination or radial (or diametric) andcircumferential post tensioning 59. Circumferential post tensioning 59creates a desirable symmetric bi-axial post compression in the slab 20.Circumferential post tensioning 59 is applied at an elevation well belowthe neutral axes of the ribs 16 thus creating eccentric post compressionin the ribs 16 and the pedestal 10 and resulting in increased nominalmoment and shear capacity of the ribs 16 as well as improvement inmulti-axial fatigue resistant of the pedestal 10, ribs 16 and the slab20. Radial or diametric post tensioning elements 58 extend from rib toopposite rib across the pedestal 10. Radial post-tensioning is appliedwith an eccentric load pattern, with higher post compression below theneutral axis of the rib. When all the prestressing elements are jacked,the foundation 100 is kept under heavy multi-axial eccentric postcompression stress, thus increasing rib structural capacity to resistsoil support reaction and providing low deflections, high stiffness andlow stress amplitudes resulting in high fatigue resistant and highdurability. Backfill is added over the foundation 100 for increasedstability and stiffness of the foundation 100.

After the concrete sets, post tensioning is carried out and thefoundation 100 is backfilled with compacted granular fill to stabilizethe foundation 100 against overturning.

Alternately the bolt assembly can be replaced by a tower section 56 bembedded in pedestal 10 concrete and the embedded section 56 b havingmeans 56 c for receiving a tower base by means of a bolted connectionarranged at the top of the section. The embedded metal cylindrical towersection 56 b encased in pedestal 10 concrete is provided with holes forrebar and post tensioning tendons 58 to extend through the metalcylinder. Post tensioning 58 tendons can extend through holes arrangedin the cylinder and across the pedestal 10, through the ribs 16 to beanchored on distal ends of the foundation.

Pedestal 10 can be any size or shape, round, triangular, square, polygonor other shape depending on the specifications of the tower and loadsthereon. The ribs 16 can be in any pattern around the pedestal 10. Inone embodiment shown in FIG. 2 the foundation 100 may have a squarepedestal 10 and ribs 16 at the corners parallel to the faces of thepedestal. The pedestal 10 may have a stepped construction with anenlarged lower cross section to reduce the length of the cantileveredribs 16.

Pre-assembled reinforcement sections (meshes) of the slab 20 componentscan be lowered into place in the slab 20 to speedup construction. Allrebar dowel or metal shear connectors extending through a constructionjoints may be galvanized or Epoxy coated to prevent corrosion. The useof mechanical couplers in the foundation 100 shall be limited oravoided. Specified mechanical couplers must be tested and certified forthe number of load cycles in the life span of the foundation 100.

In another preferred embodiment, the ribs are cast in place in reusablerib forms 16 b. The ribs 16 are cast in place jointly with the pedestal10 in one continuous pour over the slab. Optionally, the ribs 16, thepedestal 10 and the slab 20 are all jointly cast in one pour. All ribinternal components including rebar assembly with dowels andprestressing elements are placed inside the forms then cast in placeconcrete is poured into the rib forms 16 b as well as into pedestal 10and slab 20 forms.

Rib reinforcing cages can be assembled above grade and lowered into thefoundation in one or more sections.

In a preferred embodiment rib forms 16 b with internal rib reinforcingcages are preassembled and lowered into the foundation by cranes to meshwith slab reinforcing sections already placed in the foundation. Theradial reinforcing panel of the slab 20 enables the meshing rib dowelsbetween slab reinforcing without geometric interference.

Ribs 16 can also be made in segments and eventually united by meansdoweling or by using segmented post-tensioned construction techniques.Rib anchor zone with anchor trumpets and hardware can be prefabricatedseparately of higher strength concrete than the rest of the rib.

Prefabricated perimeter beams 190 with post tension ducts could serve asperimeter forms become part of the structure. An array of precast,rectangular or L-shaped beams with means for connecting to the slab 20and the ribs 16 can be used. The perimeter (edge) beams can restdirectly on the mud slab and connect to the slab 20 using horizontaldowels and shear key arranged at its inner side. Optionally theperimeter beam is elevated and connects to the top of the slab 20 usingdowels extending from its bottom. The precast perimeter beams 190 mayhave dowels and shear keys extending from their sides ends forconnecting to the ribs 16. In this case the ribs 16 will havecorresponding dowels and shear keys for receiving and supportingperimeter beams 190. The connection between ribs 16 and perimeter beams190 is established using closure pours in small cavity at theconjunctions.

In another embodiment the foundation 100 pertains to hybrid gravitybased and rock anchored foundation system. Ribs 16 can be made witharrangement, mechanisms and connecters for receiving piles 400 ormicro-piles 401 or anchors 404 in different configurations. Verticalthrough holes in the ribs 16 can provide means for receiving a pile oran anchor. Bearing elements and grouting are arranged on top of each ribto establish the required structural connection. An array of bearingplates 404 b with tensioning nuts 404 c on each soil/rock anchor is usedto compress the foundation 100 against supporting soil. Vertical throughholes with corrugations for the anchor extend through the foundation.Bearing plates 404 b with tensioning nuts 404 c can be placed on top ofthe pedestal 10 or in the foundation 100. If desired ribs 16 may havepiers extending vertically from the ribs 16 and the top of pierelevation is raised above grade to make anchor bolts accessible fortensioning and testing. Typical rock or soil anchor construction andgrouting methods can be utilized. Another option is to house rock anchorbolts and bearing plates 404 b and tensioning nuts 404 c in accessiblecorrosion protection compartments above the foundation 100.

In another embodiment the invention pertains to a foundation 100 thatcomprises the following elements:

-   -   1. A vertically extending pedestal 10 that is cast in situ, out        of concrete, the pedestal 10 serving to receive and support the        tower structure;    -   2. A substantially horizontal support slab 20 that is cast in        situ out of concrete, the support slab 20 covering an area of        ground larger than that covered by the pedestal 10;    -   3. A plurality of radial ribs 16 extending radially outwardly        from the pedestal 10 and spaced around the pedestal 10, each rib        is being joined along the base thereof to the support slab 20        being joined along an inner side thereof to the pedestal 10,        each rib has means for receiving a rock or soil anchor;    -   4. An optional plurality of perimeter beams 190, or stiffened        slab edge 190 a, spanning continuously, near the perimeter of        the foundation, between ribs 16 and supporting the slab 20 may        be employed;    -   5. An array of soil or rock anchors 404 extending through the        foundation 100, preferably through the ribs 16, may extend down        into the ground below the foundation, each anchor having a        bearing element in or above the foundation 100 and compressing        the foundation against support soil when the anchors are        tensioned.

The prefabricated components can be molded at a facility undercontrolled conditions for good quality concrete setting and controlledrebar spacing which is superior to what can be obtained on a job siteand at a lower cost. The ribs 16, acting as deep stiff horizontalcantilever support, allow the base of the foundation slabs to have arelatively small thickness using less cast in place concrete and rebarthus lowering the cost for each foundation.

Alternatively ribs 16 can have reusable temporary supports 170, or othermeans, arranged at the ribs 16 to hold the ribs 16 in place, maintainthem plumb during construction and elevate them at a predeterminedheight over slab reinforcing. This style of ribs 16 is intended to beraised above the ground or mud slab 14 so that the foundation supportslab 20 can be poured in place continuously under ribs. Dowels and shearconnectors for this style may be arranged at the bottom of the rib forconnecting with base slab 20 which extends under the raised rib. Whenthe concrete cures the continuous support slab 20, extending under theribs, is united to the prefabricated ribs 16 and the ribs 16 are alsounited to the pedestal 10. The rib inner ends will be partially encasedin the pedestal 10 to increase rib torsional end resistance. The finalresult is continuous monolithic foundation wherein loads are carriedacross the structure vertically and laterally through the continuousstructure by the doweled and spliced reinforcing steel bars which areintegrally cast into the pedestal, ribs 16 and support slab 20. Thecombination of the high stiffness of the ribs 16, solid pedestal 10 andcontinuous slab 20 construction across the pedestal 10, and under ribs16, allows the slab 20 to behave structurally as a continuous slab 20over multiple rigid supports resulting in small flexural and shearstresses in the slab 20, reducing deflections, improving fatigueconditions and increasing the stiffness of the foundation as well asallowing for the benefits of an economical design.

Cast in situ concrete can be shielded from extreme weather, includingheat, cold, rain and snow, by simply extending blankets, covers orshields between ribs 16 during construction, and then using heaters orfans as required to regulate temperature, humidity of concrete to allowfor proper setting and curing conditions.

Another embodiment of the present invention pertains to a levelingtechnique that simplifies the tower base leveling process and shortensthe number of steps required for grouting under a tower base. The bolttemplate is provided at the very top of the bolt assembly with at leastthree sets of additional bolts and corresponding threaded bolt insertssuitable for embedment in the concrete. Such leveling bolts 53 andinserts 53 b will be located outside or inside the bolt circle of towerbase, but directly under tower base flange. This allows for continuityof grout bed construction and provides an easy access to leveling bolts53. Small cutouts at leveling bolt locations may be used. Anotherbenefit of this leveling technique is having the ability to applycontinuous grout bed that is free of cold joints, under tower baseflange in one session as well as having the ability to tension allanchor bolts in one work session.

The foundation design can be reconfigured to support lattice towerscomprising multiple columns connections to foundations in a spacedarray. The ribs 16 will be provided with column receiving componentsincluding embedded anchor bolts (or grouting around embedded element)and an integrated pier design into the rib. The rib geometry may bewidened and enlarged at the integral pier. The array of said integratedpiers ribs 16 are fitted with means for receiving and supporting thelegs or the columns of the lattice tower 200. The integrated piers canextend above final grade elevation, while the top of pedestal 10 maystay below final grade elevation. For this foundation style, pedestalelevation may be depressed and tower receiving components may not berequired in the pedestal 10. This configuration may also be used inoffshore applications wherein a prefabricated gravity foundation 100 isconnected to lattice tower structure 200 that is fitted with a windtower receiving component at its top. The foundation 100 will beinstalled over prepared seabed and filled with a suitable backfillingmaterial 13, and surrounded with scour protection 13 b.

In permafrost conditions, the foundation 100 may be supported on anarray of concrete piers deeply embedded and frozen into the ground.Anchors can be used to secure the ribs 16 to their supporting piersaround the perimeter of the foundation. If a slab 20 is incorporated inthe design, the slab 20 bottom elevation may be set above gradeelevation.

This invention pertains to a fatigue resistant gravity based spreadfooting for use under heavy multi-axial cyclical loading of a wind tower300 which comprises a plurality of components, namely a central verticalpedestal 10, a substantially horizontal continuous bottom support slab20 with stiffened perimeter, a plurality of radial reinforcing ribs 16extending radially outwardly from the pedestal 10 and athree-dimensional network of vertical, horizontal, diagonal, radial (ordiametric) and circumferential post-tensioning elements that keep thestructural elements under heavy multi-axial post compression withspecific eccentricities and orientations that are intended to reducesstress amplitudes and deflections and allows the foundation 100 to havea desirable combination of high stiffness and superior fatigueresistance while improving heat dissipation conditions duringconstruction by having a small ratio of concrete mass to surface areathus eliminating the risk of thermal cracking due to heat of hydration.

Vertical prestressing of the pedestal 10 can be carried outindependently of tower receiving elements. A pedestal 10 may have anarray of vertical post tensioning elements 56 that does not connect to atower 300, and an embedded tower section 56 b bolted to a towerstructure 300.

Radial post-tensioning 58, extending across the foundation 100, in pairsof ribs 16, allows for the desirable structural continuity and thedirect transfer of loads from a downwind ribs 16 into the pedestal 10and then into the opposing upwind ribs 16. Radial and circumferentialpost compression stresses in the slab 20 and/or perimeter beams 190allows for a desirable reduction in stress amplitudes the structuralcontinuity between slab 20 spans and/or perimeter beam 190 spans, acrossthe ribs 16, thus creating a desirable load sharing mechanism betweenadjacent ribs 16 by forcing more ribs 16 to be engaged in resistingtower loads.

The invention pertains to a durable, high-stiffness, fatigue-resistantfoundation structure 100 for onshore wind tower installations whichcomprises:

-   -   1. a central pedestal 10 that is made of cast-in-place concrete        with concentric vertical prestressing elements and eccentric        multi-axial horizontal and/or radial post-tensioning elements;    -   2. an array of cast-in-place eccentrically post-tensioned radial        ribs 16;    -   3. a cast-in-place slab 20 with heavily post-tensioned thickened        slab edge 190 a.        All components are made of high strength reinforced concrete and        are rigidly connected to each other to behave as a monolithic        spread foundation structure. The structural components are        rigidly connected with arrays of rebar dowels (passive        reinforcing) or post-tensioning elements extending through the        conjunctions. The slab 20 functions as a two-way slab system        that is free of construction joints across the footprint of the        foundation and spans continuously over multiple ribs 16.        Perimeter post tensioning 59 a or circumferential post        tensioning 59 of the slab 20 is applied at an elevation well        below the neutral axes of the ribs 16 to cause eccentric loading        of the ribs 16 and the pedestal 10. Radial post-tensioning        elements with an eccentric load pattern, with higher post        compression at the bottom of the rib, extend from rib end to        opposite rib across the pedestal 10, or to the opposite end of        the pedestal 10. When all the prestressing elements are jacked,        the foundation 100 is kept under heavy multi-axial eccentric        post compression stress, thus increasing rib structural capacity        to resist soil support reaction and providing low deflections,        high stiffness and low stress amplitudes resulting in high        fatigue resistant and high durability. Backfill 13 is added over        the foundation 100 for increased stability and stiffness of the        foundation 100.

Soil support reaction under the slab 20 is transferred from the slab 20to the ribs 16 and thickened slab edge 190 a (or perimeter beams 190) asin two-way slab systems with more load distribution going to the ribs 16in the primary span. Perimeter or circumferential post-tensioning areapplied, perpendicular to the ribs 16, in the orientation of the primaryspan that effectively reduces stress amplitudes and deflections in theslab 20 by keeping the slab 20 under heavy post-compression in thedirections of primary slab spans around the foundation. The size,distribution, eccentricity and location of post tensioning elements 58in the ribs 16 and the slab 20 are used, by the engineer, to dictate thenatural frequencies of the foundation 100 to be in a safe range relativeto operating frequencies of the wind generator according to turbinemanufacturer recommendations.

The said vertical, radial and circumferential post-tensioning in thefoundation keep all the structural components (Pedestal 10, ribs 16,slab 20, thickened slab edges (or integral edge beams)) undermulti-axial post compression confinement resulting in lower stress rangeamplitudes thus yielding higher stiffness, more effective crack control,lower deflections and improved fatigue resistance. Superior fatigueresistance and long life-span are achieved by keeping most of thestructural elements of the foundation 100 under multi-axial compressionwhile resisting operating loads or even during normal and abnormalextreme loads from the supported structure (wind power generator).

In a preferred embodiment, rib post-tensioning requirement are reducedby engaging fully developed bar dowels from the rib into the pedestalconnection as well as extending fully developed radial rebar dowels ofthe slab 20 into the pedestal 10, thus allowing passive reinforcing toparticipate in the said connection especially under extreme loads.Radial slab reinforcing pattern with tapered rib width was found to bevery cost effective as the rib to pedestal connection benefits from alarge number of top and bottom radial slab reinforcing barsparticipating in the said connection, as the rib width widens, thusreducing the number of bottom post-tensioning strands required for thesaid connection.

The structural configuration of the foundation 100 reduces the overallcumulative deflections in the structure under tower loads andsignificantly improves the rotational stiffness of the foundation 100which is a key factor in determining the size of foundations in windturbine installations. The rotational stiffness is also improved by theinterlocking between surrounding soil (after backfilling) and themultiple surfaces and vertical faces of the foundation structure. Thehorizontal stiffness is improved by the passive earth pressure on themultiple faces of the structure. Both rotational and horizontalstiffness achieved by this design are much higher than conventionaltapered inverted-T gravity spread footings especially for onshorefoundations installed below grade in an excavated pit because of theincreased interlocking surface area and increased passive earth pressureand increased friction on the multiple faces of the fatigue resistantfoundation 100.

The solid-core pedestal 10 comprises a continuous reinforcing cage and atower receiving component, such as anchor-bolt assembly, with acylindrical array of bond protected high strength post-tensioning bolts,for connecting to wind tower base flange 301. In another embodiment andthe tower receiving component may comprise an embedded cylindrical metaltower section 56 b with means 56 c for connecting to a tower sectionsuch as a flange with bolt holes for receiving bolts at its top and withan array of holes to allow the passing of rebar and post tensiontendons. The anchor bolt assembly ensures structural continuity betweenthe tower 300 and the pedestal 10. The post-tensioning forces of theanchor bolts are selected by the engineer to insure that the tower baseflange 301 remains in contact with the pedestal 10 under extreme normaland abnormal load conditions. The bolt assembly includes, at its bottomend, a bearing element that may consist of an embedment ring plate thatis made of segments that are welded together.

Radial post-tension tendons and rebar reinforcing elements extendingfrom the ribs 16 and the slab 20 pass through the pedestal reinforcingcage, or through holes in the embedded metal tower section.

As shown on the drawings, post-tensioning elements are flaredhorizontally, profiled vertically, arranged in matrix groups, spaced anddraped in a manner that allows for optimum utilization ofpost-tensioning and ease of installation while avoiding tendoncongestion and stress concentrations as tendons crisscross in thepedestal 10. The regrouping of tendons to form flat and wide matrixalong each axis was found to be effective in avoiding tendon congestionespecially in the pedestal 10. The said flat and wide matrix of tendonsis placed as high or as low as possible to maximize their moment armsand optimize their contributed moment capacity. For corrosionprotection, bonded (multi-strand and grouted) or un-bonded encapsulated(mono-strand) post-tensioning elements and their associated constructiontechniques can be used in the foundation 100.

The rib's thickness can be gradually increased at the connection to thepedestal 10 to increase rib flexural, shear and torsional capacity andenhance pedestal confinement. The post-tensioning requirements can bereduced by engaging dowels at rib-to-pedestal connection and byextending fully developed radial dowels from the rib and the slab 20deep into the pedestal, thus allowing passive reinforcing to participatein the connection.

In another embodiment, ribs 16 top surface can be tapered to asubstantial slope extending vertically to an elevation near the top ofpedestal allowing the ribs 16 to benefit from diaphragm action at theirinner zone and also provide lateral support for the full height of thepedestal 10 and to provide concrete confinement at the highly stressedzone at the top of pedestal under tower base flange 301.

The foundation may have a circular or polygonal foot print. Thethickened slab edge 190 a 190 a (perimeter beam) may extend above orbelow the foundation. A shallow perimeter beam profile should beselected for ease of backfilling and improved accessibility for rollercompactors during the backfilling of the foundation 100. A thickenedslab ring beam 190 a may be designed to be at an offset distance awayfrom the slab edge allowing the slab segment, outside the ring, tobehave as a cantilever. This configuration reduces slab span anddeflections as well as the volume of concrete required in the foundation100.

As shown on the drawings the size of the slab 20 and its continuousreinforcing including that of the thickened slab is configured to createa rigid composite connection to the ribs 16 with high stiffness which issufficient to allow adjoining ribs 16 to participate more in resistingthe loads and thus reducing local deflections and increasing overallfoundation stiffness in addition to reducing the unsupported length ofcantilever radial ribs 16.

In a preferred embodiment, as shown on the drawings, the pairing of theribs 16 on distal ends and the continuous perimeter beam constructionyield a cost effective layout of post-tensioning that uses a smallnumber of tendons and corresponding anchors as well as reduces frictionlosses by avoiding sharp turns in tendon layout. The tendons of the ribs16 are anchored in a matrix array at the outer end of the rib and extendhorizontally and diagonally along the rib to split into at least twogroups 58 a and 58 b one near the bottom and the other near the top ofthe rib as it connects to the pedestal 10. The tendons are moreconcentrated at the bottom than at the top in a concentric prestressingpattern that is intended to maximize the structural capacity of thefoundation and meet the flexure and shear demand of the governing loadcases.

Ribs 16 may have thickened flanges, at their connection to the pedestal10, that may also house post tensioning anchors for tendons 58 extendingfrom ribs 16 on the opposite side of pedestal. The ribs 16 may also havepost tensioning anchors along their sides or tops if tendon curtailmentmethods are applied in the design. The ribs 16 may also have embeddedloop anchors if looping of tendons is used in the design. Loop anchorscould also be used in the pedestal 10 to support precast concrete towers300 b.

As shown on the drawings the tendons in ribs 16 extend horizontally anddiagonally to be split into three distinctive groups as they enter thepedestal. The first group 58 a with more tendons is placed at the bottomof ribs 16 or in the slab to create camber for reducing deflections andimproving foundation soil contact as well as meet the high flexuraldemand from the governing load cases, and the second group 58 b slope updiagonally to follow the geometry of the top of the rib as they enterthe pedestal 10. The third group 58 c is in the middle and it startshorizontal at rib anchor block and diagonally slopes down towards thebottom of the rib to enter the pedestal 10 for optimum use of thetendons. Tendons in the pedestal 10 are fanned and flared into groups tosimplify the installation and maximize their utilization by increasingtheir moment arms measured from the top or the bottom of the structuralconcrete. Additional post-tensioning groups for shear resistance can beprovided by providing tendons that traverse the shear failure plane inthe ribs 16.

In another embodiment the post-tensioning in the ribs 16 consist ofthree distinctive groups:

-   -   1. A bottom group 58 a that is horizontal at the bottom of the        rib 16 and in the slab 20 and may be grouped with slab post        tensioning,    -   2. A top group 58 b that is diagonally sloped upward to follow        the geometry of rib top,    -   3. An optional middle group 58 c that starts horizontal at rib        outer edge and is diagonally sloped down towards the bottom of        the rib to eliminate dead load deflections and keep the ribs 16        and pedestal under post compression during normal operating        conditions and also provide the high demand of post-tensioning        capacity required at the bottom of the rib for downwind load        cases, and traverse the shear failure plane for ribs 16 in the        governing downwind load cases and provide additional shear        resisting capacity in each rib,        such that the number of strands in the bottom of the rib and the        pedestal 10 is much higher than that at the top thus causing a        multi-axial, heavy, eccentric horizontal post compression in the        foundation after the tendons are jacked.

Anchor-blocks for perimeter or circumferential post-tension tendons canbe placed at perimeter beams 190, (ring beams) or thickened slab or atthe edge of the foundation or on top of perimeter beams 190 or on sidesof ribs 16. A preferred layout with two anchor blocks on opposite sidesof the foundation and with semi-circular (180-degree) tendon arrangementis shown on drawings. Ring tendons with ring anchors 59 b (such asdog-bone anchors) can be used to avoid having blisters on the foundation100. Styrofoam block-outs 59 c can be placed in the foundation 100according to anchor manufacturer recommended dimensions. When theconcrete reaches the sufficient strength ring tendons are jacked andring anchors grouted.

The foundation is made of a network of prestressed concrete elementsthat can be structurally analyzed, with the strut and tie method, as toa three-dimensional structure made of an array of vertically andhorizontally oriented truss-girders joined at the center, with majortension chords reinforced with prestressing tendons, based on bothupwind and downwind load cases wherein tension forces in the structureare resisted largely by prestressing elements and passive reinforcingand compression forces are resisted largely by the concrete elements.The structure can be analyzed as a circumferential array of verticallyoriented trusses that are fixed at their inner ends to the centralpedestal 10 and are laterally stabilized at their bottom by a horizontaltrussed diaphragm formed by perimeter post tensioning 59 a, in the slabor perimeter beam, and radial bottom tendons 58 in the ribs 16 or theslab 20.

In another embodiment the fatigue resistant foundation 100 comprises acircumferential array of vertically oriented eccentrically prestressedcantilevered girders that are fixed at their inner ends to a centralpedestal 10 that is laterally supported and confined through most of itsheight by rib concrete, and the ribs 16 and pedestal 10 are laterallystabilized at their bottom by a horizontal prestressed concrete trusseddiaphragm, with a continuous slab 20, and the prestressing is providedby radial tendons in the ribs 16 (or the slab 20) and circumferentialpost tensioning elements 59. The radial and circumferential tendonsprovide eccentric prestressing in the ribs 16 and the pedestal 10. Thepedestal is vertically prestressed and is structurally fixed to a towerbase 301 of a pylon.

In a preferred embodiment the construction of the foundation 100 mayutilize pre-assembled perimeter beam reinforcing cages, built insegments with overlapping spliced bars at their ends, and each having anarray of shear resisting vertical ties and flexure resisting horizontalbars as well as local reinforcing at anchor locations as shown on FIG.X.

As shown on the drawings, the foundation has specific reinforcinggroups. The ribs 16 have flexure reinforcing concentrated at the bottomand the top, vertical stirrups for shear reinforcing that are tightlyspaced at high shear zone along rib inner end, rib skin reinforcing onach face and bursting and splitting reinforcing made of horizontalhairpins extending between the said rib skin reinforcing, as well asstraight or U-shaped horizontal dowels for embedment into the pedestal10 and vertical dowels, at the bottom, for composite action with theslab 20. As shown on the drawings the vertical stirrups also function asdowels for composite action of the slab 20. The said dowels are spacedsuch that they could mesh between slab reinforcing bars withoutgeometric interference, if the rib reinforcing is built in preassembledcages and placed over the slab reinforcing. In order to maximize shearcapacity vertical stirrups are placed side-by-side, in pairs, at theinner rib zone where the shear demand is high.

Anchor zones are provided with heavy reinforcing with trim bar and ties.The ribs 16 may also have horizontal reinforcing dowels, perpendicularto the ribs 16, to facilitate the structural continuity of the supportedperimeter beams 190 or the thickened slab, across the width of the rib,by means of splicing the said dowels with perimeter reinforcing.

The pedestal 10 has a horizontal mesh at the top and skin reinforcing atall faces as well as at least one cage, around the anchor bolt assembly,comprising vertical tightly meshed anti-bursting reinforcing includingtwo cylindrical meshes confining the anchor bolts each comprisinghorizontal hoops and either C or Z-Shaped bars and a radial array ofhorizontal hair-pins or stirrups tying both cylindrical meshes orspirals stirrups each housing a number of anchor bolts. The pedestal 10cage assembly may comprise two concentric tightly meshed cagessurrounding the anchor bolts one from the inside and the other from theoutside with radial array of anti bursting and splitting resistanthairpins extending between the two cages # and #. Additionally an arrayvertically oriented pedestal vertical anti bursting and splittingresistant reinforcing group, comprising circumferentially spacedvertical hairpins extending between said top horizontal mesh and ahorizontal bottom reinforcing mesh in the pedestal 10 or slab 20, isincluded in the pedestal cage. The vertical hairpins # also function assupports to secure tendons in the pedestal 10 during construction.

Upper and lower slab reinforcing meshes may have any pattern such as asquare grid, a circular array with radial pattern or overlappingpie-shaped segments. Additionally, an array of slab reinforcing locallyarranged beneath the ribs 16 and being oriented parallel to the ribs 16and extending into the pedestal 10 to facilitate composite action. Theslab 20 may also be reinforced with post-tensioning elements in anypattern including radial, circumferential, perimeter or a square grid.

The foundation system relies on the use of many prefabricated componentsincluding rebar meshes and cages, pedestal cage assembly, pre cutpost-tensioning strands, preassembled post-tensioning bundles, pre-cutpost-tensioning duct sections and prefabricated concrete forms.

Reusable rib forms 16 b are utilized to form foundation perimeter, theribs 16 and the pedestal. Forms can be made to be segmented, universal,expandable and adjustable to work for different foundation sizes. Asshown on FIG. X rib forms 16 b can be made with adjustable supports toelevate the forms above the wet slab concrete during construction if thefoundation is built in one pour. Rib forms 16 b may sit directly on thehardened concrete slab 20 if the foundation is built in two pours. Ribforms 16 b may be made with two side-panels of stiffened non-stickplates and an array of adjustable horizontal spacers between the panelsto maintain proper geometry and resist the lateral pressure of wetconcrete. Rib and pedestal forms 102 may be fitted with lifting lugs ormeans for receiving and supporting ladders, catwalks and work platformsto allow for access around the foundation. The forms may have means forsecuring post-tension anchors and hardware at specific spacing duringconstruction. The forms may also have means for hanging an supportingrib reinforcing cages.

The foundation 100 may be supported on piles, or micro-piles 401 orpiers 402 or rammed-aggregate piers 405. The foundation 100 may receiverock anchors 404 or soil-anchors in a conventional manner. Aconstruction site is prepared by excavation and flattening andpreparation of soil for the foundation. The foundation 100 may be set ona mud slab or on compacted granular fill. The mud slab is a thin plainconcrete layer intended to provide a clean and level base for foundationinstallation.

In one embodiment, After the foundation site has been prepared, slabreinforcing is placed inside slab forms and the slab is poured in placewith dowels extending up from the slab 20 to receive ribs 16 andpedestal in a second pour. Rib and pedestal rebar and cage placementwith post-tension tendons (or duct) placement are accomplished and formsare installed in place and a second pour is carried out. Alternativelythe foundation 100 can be poured in a single pour with the use ofaccelerators in the concrete mix and by following a well designedconcrete pour sequence. A set of small footings, placed within the mudslab, can be used to support and elevate the rib forms 16 b and pedestalforms 102 during construction. Slab 20, pedestal and rib reinforcingelements are assembled in the foundation 100. Forms are placed in thefoundation around the perimeter, the ribs 16 and the pedestal and theconcrete is poured into the foundation in a carefully designed poursequence. One option is to start with slab 20 and the bottom part of theribs 16 and the pedestal with accelerator in the concrete mix to sealthe bottom of rib and pedestal forms 102 by the time the slab 20concrete is finished, the ribs 16 and the pedestal are poured jointly insmall lifts.

When the concrete hardens to certain strength, post-tension elements arejacked and grouted as required. The tower base flange 301 is thenattached to the pedestal 10 and grouted, and the tower anchor bolts aretensioned after the grout reaches sufficient strength.

In a preferred embodiment, the invention relates to a high stiffness,fatigue resistant, wind turbine foundation 100, supporting a windgenerator with a multi-megawatt rating and subjected to extremely highcyclical upset loads that comprise the following components:

-   -   1. a substantially massive and wide central pedestal 10 with        substantially solid core concrete construction that is kept,        through most of its height, under a combination of lateral        structural concrete supports and confinement, high vertical        post-compression stress and high eccentric multi-axial lateral        horizontal post-compression stress across its width, provided by        said lateral supports and post-tensioning elements that traverse        the width of the pedestal 10, through non-segmented concrete        construction, along multiple axes in a concentric pattern, and        having a set of upright, circumferentially spaced anchor bolts,        for providing the said high vertical post-compression stress,        extending through said pedestal 10, and having lower ends        anchored relative to an anchor ring and upper ends projecting        upwardly from said top end of said pedestal, said anchor bolts        being substantially bond protected along their length, said        upper ends of said bolts project upwardly from the said pedestal        10 through a base flange of an annular tower structurally fixed        atop the said pedestal 10, and also having an upright heavily        reinforced cage of tightly meshed rebar, and concentrically        arranged around both sides of the anchor bolt cage with opening        to allow the passing of lateral load transfer elements,    -   2. a support slab-on-grade 20, cast-in-situ out of concrete        against the soil, in an excavation pit, of continuous        construction and covering a footprint substantially larger than        that of the pedestal 10 and having a thickness that is much        smaller than the depth of the pedestal 10 and having thickened        edge made of concrete integral with the support slab 20 and        having horizontal post-tensioning elements to keep the slab 20        under heavy multi-axial post compression,    -   3. an array of concentrically arranged ribs 16 made of deep        girder construction, integral with the pedestal and support slab        20, and jointly cast-in-situ with said pedestal 10, and        extending vertically, above the slab 20, to an elevation near        the top of pedestal 10 such that the pedestal 10 is laterally        supported and substantially confined below the said tower base        flange 301, and having a width that is substantially smaller        than that of the said pedestal, and being arranged such that        pairs of ribs 16 outwardly extend from opposite sides of the        pedestal with post-tensioning elements inwardly extending from        the distal ends of the ribs 16 through the pedestal,    -   4. reinforcing rebar and prestressed dowels extending from the        ribs 16 deep into the core of the pedestal 10 from distal ends,        and arrays of dowels, made of rebar, extend between the slab 20        and each of the ribs 16 and the pedestal along their        conjunctions,    -   5. a suitable backfill material 13 placed over the foundation        100, to stabilize the foundation 100 against overturning,        followed by tower base installation and grouting,        the foundation 100 is kept under heavy multi-axial        post-compression such that tower loads are resisted by pairs of        ribs 16, on distal ends of the pedestal 10, wherein each pair of        ribs 16 form a high stiffness continuous, non-segmented,        laterally supported, post-tensioned girder extending between        distal ends of the foundation 100 with continuous uninterrupted        composite action from the slab-on-grade 20.

In another embodiment, slab post-tensioning can be arranged at anycombination of perimeter, radial or diametric, or other patterns.

In another embodiment, composite action is further facilitated withradially oriented, reinforcing bars locally arranged in the slab 20,beneath the ribs 16, and extended deep into the pedestal 10, in additionto an array of vertical dowels extending between the rib and the slab 20that function as shear connectors.

In a preferred embodiment, the invention pertains to a foundation 100for supporting a wind generator with a multi-megawatt rating andsubjected to extremely high cyclical upset loads, with increasedstiffness and improved fatigue resistant comprising:

-   -   1. a support slab-on-grade 20 of non-segmented continuous        construction with a circular integral perimeter beams 190 with        circumferential post tensioning elements 59 made of two        180-degree tendon segments forming a 360-degree circle, with        anchors at the opposite sides of the foundation,    -   2. a central cylindrical pedestal 10 integral with the support        slab-on-grade of solid non-segmented construction and having        vertical post-tensioning elements,    -   3. ribs 16 integral with the support slab and the central        pedestal 10, on top of the slab 20, with three or four pairs of        ribs 16 radially extending from opposite sides of the pedestal        10 and post tensioning elements extending axially and diagonally        from anchors placed at the distal ends of the ribs 16 through        the pedestal 10,        such that the ribs 16 and the perimeter beams 190 function as a        prestressed trussed diaphragm structure with infill panels, and        pairs of ribs 16 on distal ends of the pedestal 10 function        continuous post-tensioned girder, that is free of construction        joints, with continuous composite action from the slab 20 and        the foundation 100 is kept under eccentric multi-axial        horizontal and concentric vertical post-compression, and        circumferential post-tensioning in the slab 20 effectively        reduces stress amplitudes and deflections in the slab 20 by        keeping the slab 20 under heavy post-compression in the        direction of primary slab spans which is roughly perpendicular        to the ribs 16.

In a preferred embodiment, the rib extends vertically from the bottom ofthe foundation to an elevation near the bottom of the tower base flange301 to enable the ribs 16 to participate in resisting bearing loadsunder the tower base flange by increasing the area of the cross-sectioninvolved in bearing resistance under the tower base flange 301 andincreasing the permissible bearing strength under the base flange or thegrout bed by increasing the bearing area measured at the surroundingfaces of the concrete. The geometric configuration and the improvementin bearing resistance, in this invention, allow the engineer to specifyconcrete with only one relatively low compressive-strength for theentire foundation structure. In contrast, high bearing stresses underthe tower base flange 301 in conventional gravity spread footings, forcethe engineer to specify concrete with higher compressive strength forthe pedestal and a lower compressive strength for the base.

The proximity of inner rib ends to the tower base flange 301 allows theinner zones of the ribs 16 to remain under vertical compression stressescaused by vertical post-tensioning forces between embedment ring 54 andtower base flange 301. The said vertical compression stress zonesimproves confinement conditions and fatigue resistance in rib innerzones.

Bonded and grouted multi-strand system was found to be expensive andlengthy and requires an additional step of grouting and may not beeconomical for some onshore installations. It is preferable to useun-bonded, encapsulated mono-strands, arranged in bundles and installedin the foundation reinforcing prior to concrete casting, which wouldreduce construction costs and improve construction schedule.

In a preferred embodiment post-tensioning in the foundation 100 is madeeccentric, to create cambers in the foundation 100 that could result inreduced deflections and improved foundation-soil contact. As an example,the eccentric prestressing of the ribs 16 creates a convex shaped camberin the foundation 100 that helps reduce the deflections under turbineweight and operating loads. Similarly cambers can be used in perimeterbeams 190 and slab sections to reduce slab deflections and improvefoundation-soil contact conditions by ensuring a more uniform bearingpressure under the foundation.

The vertical profile (elevation) of circumferential tendons in thefoundation 100 may be adjusted at mid spans and under supporting ribs 16to optimize their utilization.

In another embodiment gradual transition of geometry at the conjunctionof the structural elements is employed to prevent stress concentrationand fatigue related problems. As an example the use of fillets andcurved transition is desirable at the conjunctions between ribs 16,pedestal 10 and the slab 20.

In a preferred embodiment, as shown on FIG. X, the inner ends of theribs 16 are tapered to become wider as the rib connects to the pedestal,in order to satisfy the high flexural, torsional and shear demands atthe inner zone of the ribs 16, and to distribute the multi-axialcompression over large surface area to help reduce splitting andbursting reinforcing at the sides of the pedestal 10.

In another embodiment low relaxation post-tensioning strands are used toreduce post tension losses over time. Concrete accelerators andplasticizers and otter admixtures may be utilized in the concrete mixdesign. The small thickness of the structural elements may allow foron-site steam curing of concrete.

A hollow pedestal 10 cross-section may be used, however it can beproblematic. Hollow pedestal above the frost depth where there iselevated water table is problematic. Hollow pedestal results in reducedstiffness, stress discontinuity, indirect load transfer and stressconcentration with local deformation in the shell resulting in poorperformance under heavy flexural and torsional loads from thecantilevered rib, as well as additional problems stemming from stressescaused by frost forming and freezing of ground water in the foundation.

In another embodiment the cross-section of the rib may change anddimensions along its length may change. For example, the section maystart rectangular and gradually a top flange may be enlarged to reducestresses in the upper zone of the rib.

In another embodiment the pedestal 10 may have an enlarged cross-sectionat the top followed by a transition into a smaller cross-section below.The upper enlarged cross-section may help with improve bearing strengthat the top of the pedestal below tower base flange 301, or bearingwasher plate, and the high strength grout bed according to AmericanConcrete Institute design guidelines.

The present invention pertains to a foundation design that overcomes thethermal cracking problem stemming from heat of hydration, in largefoundation pours for large multi-megawatt rated turbines, by using astructural configuration coupled with post-tensioning techniques thatreduce the thickness of the structural elements, while increasing thesurface area of concrete pours, thus improving heat dissipationconditions and causing a the ratio of concrete mass to surface area tobe roughly 40% to 50% less than in conventional design for foundationsfor the same turbine under the same loading and geotechnical conditions.

The present invention uses a tower base leveling and grouting methodwithout using tower anchor bolts for leveling, or having to use levelingshims which cause undesirable stress concentration at shim locationswhich could lead to localized fatigue failure at shim locations. Thistask is achieved by providing the bolt template at the very top of thebolt assembly with at least three sets of additional bolts andcorresponding threaded bolt inserts suitable for embedment intoconcrete. The said leveling bolts 53 and inserts 53 b are be locatedoutside or inside the bolt circle of tower base, but directly undertower base flange. This allows for continuity of grout bed constructionand provides an easy access for leveling bolts 53. Small cutouts atleveling bolt locations connected can be used. Another benefit of thisleveling technique is having the ability to apply grout a continuousgrout bed that is free of construction joints, under tower base 301 inone session and to also have the ability to tension all anchor bolts inone session.

The present invention improves safety and accessibility aroundfoundations during construction, and reduces hazardous conditions forconstruction crew. That goal is achieved by using reusable form sectionsthat are fitted with platform sections for forming a access platformsaround the foundation, and connect to at least one access ramp extendingbeyond the edge of the foundation. The platform and the ramp are fittedwith slip-resistant walking surface and elevated ramps all provided withguardrails and designed to applicable industry safety standards. Therelatively thin slab thickness minimizes the risk of worker injuryduring construction.

Transformer pad can be supported on precast concrete posts extendingvertically from the foundation.

Pedestal forms 102 will have openings for running electrical andcommunication conduits thus preventing problems stemming from randomlyplacing the conduits in areas that could compromise the structuraldesign.

The ribs 16 may have means for receiving and supporting prefabricatedtrays (or electrical duct banks) for housing power and communicationcables.

This foundation design can also be adapted for offshore wind turbineprojects. In this case the foundation 100 may be assembled on a barge ordry dock then transported or floated to its destination, then loweredinto a prepared seabed location. The foundation can be weighed down inplace by backfilling it with suitable material. The offshore foundation100 may be configured to receive any type of offshore piers 404, suctionpiers 403, piles 400, micro-piles 401, anchors 404 or any combination ofthe above.

The invention relates to an offshore concrete foundation 100 with highstiffness and improved fatigue resistant comprising:

-   -   1. a support slab-on-grade 20 of non-segmented continuous        construction covering the entire footprint of the foundation and        having (horizontal) diametric and perimeter post-tensioning        elements,    -   2. a central pedestal 10 integral with the support slab-on-grade        20 of solid non-segmented construction and having vertical        post-tensioning elements and also having reinforcing elements of        rebar to carry loads diametrically across the pedestal 10;    -   3. a cylindrical or conical stem 11 extending vertically above        the pedestal 10 and being fixed to the pedestal 10, and having a        hollow cross section, of equal size or smaller than that of the        pedestal 10, and may be constructed with segmented or        non-segmented construction methods and could be made with        typical cast in place over the pedestal 10 by using typical        construction methods for tall cylindrical concrete structures        such as continuous forming, successive pours, segmental        construction with precast concrete panels or other known        construction methods used conventionally for cylindrical        concrete structures such as chimneys, and the stem 11 is kept        under heavy concentric vertical post-compression stress by an        array of circumferentially arranged vertical post-tensioning        elements, and the stem 11 may have an ice cone 11 b integral        with the top of stem 11, and the stem 11 having means for fixing        a tower base 301 of a wind tower 300, the stem 11 and the ice        cone 11 b are vertically and circumferentially prestressed with        vertical and circumferential post tensioning elements,    -   4. ribs 16 integral with the support slab 20 and the central        pedestal 10, on top of the slab-on-grade, with pairs of ribs 16        radially extending from opposite sides of the pedestal 10 with        post-tensioning elements extending radially and diagonally from        the distal ends of the ribs 16 through the pedestal 10 and        keeping the ribs 16 and the pedestal 10 under heavy eccentric        post compression stress and reinforcing dowels extending from        the ribs 16 into the pedestal 10 and spliced with pedestal 10        reinforcing,    -   5. deep perimeter beams 190 extending continuously around the        foundation, made of concrete integral with the support        slab-on-grade 20 and the ribs 16 and having continuous perimeter        or circumferential post tensioning elements,        When the concrete sets, the said post-tensioning elements are        jacked and the anchor bolts are post-tensioned the foundation is        kept under heavy multi-axial post-compression.

The offshore foundation 100 is constructed on a barge or in a dry dockand then floated or transported to an offshore installation site andlowered to be places over a prepared sea bed, a suitable backfillmaterial 13 placed over the foundation 100 to stabilize the foundationagainst overturning. Scour protection measures 13 b are provided aroundthe foundation. The foundation is built with marine cement and marinegrout and is kept under heavy multi-axial horizontal and verticalpre-stress using bonded and grouted post tensioning systems rated fordouble corrosion protection and suitable for marine environment.

An offshore foundation for wind turbines comprising the followingelements:

-   -   1. A vertically extending pedestal that is cast in situ, on a        barge, out of concrete, the pedestal has an integral long stem        11 for receiving and supporting a tower structure;    -   2. A substantially horizontal support slab 20 that is cast in        situ, on a barge, out of concrete, the support slab 20 covering        an area of ground larger than that covered by the pedestal 10;    -   3. A plurality of radial ribs 16 extending radially outwardly        from the pedestal 10 and spaced around the pedestal 10, each rib        being prefabricated and being joined along the base thereof to        the support slab 20 when the support slab 20 is cast in situ and        being joined along an inner side thereof to the pedestal 10 when        the pedestal 10 is cast in situ;    -   4. A plurality of prefabricated perimeter beams 190 spanning        continuously, near the perimeter of the foundation, between ribs        16 and supporting the slab 20;    -   5. Backfill 13 for weighing down the foundation, resisting tower        loads and providing scour protection 13 b.        when the concrete sets, the precast components will become        integral with a cast-in-place components. Radial post-tensioning        tendons extend from rib ends opposite rib ends across the        pedestal 10. Vertical post-tensioning is arranged in the        pedestal 10 as well. The stem 11 and the ice cone 11 b may also        benefit from circumferential post-tensioning.

The pedestal 10 has means for receiving and supporting a tower 300 orpylon. The upper portion of the pedestal 10 (the stem 11) may be made inmultiple consecutive cast in situ pours, depending on its height.Alternatively, the stem 11 may also be made by joining precast segmentswith circumferential and vertical post-tensioning to form the stem 11.

In another embodiment of the invention, a wind turbine is fabricated ona barge with precast concrete element as following. The barge surface isprepare with a non bonding agent or a thin membrane at the foot printwhere the foundation to be built. Lower slab reinforcing mesh sectionsare assembled and placed and the pedestal cage reinforcing is assembledat the center of the foundation. Upper slab reinforcing mesh sectionsmay follow after slab post tension duct is placed. Precast concrete ribs16 are placed in a radial array around the pedestal cage and precastconcrete perimeter beams 190 are arranged around the perimeter of thefoundation. Post tensioning ducts in the pedestal space and at perimeterbeam-to-rib connections are placed to pair with corresponding duct inthe precast members. Forms for pedestal and for closure pours atrib-to-perimeter beam connections are installed. Slab concrete is pouredfollowed by pedestal 10 concrete and closure pours at rib-to-pedestalconnections. Stem 11 is fabricated possibly in multiple consecutivepours depending on pedestal height. Stem 11 design may incorporate anice cone 11 b at its top. Post tensioning tendons are installed, thejacking and grouting of tendons is carried out. Some pylon sectionscould be installed earlier prior to transportation. The finishedfoundation 100 is transported to its offshore installation site using asuitable means of transportation such as towing the barge.

In another embodiment of the invention relates to an offshore foundationfor wind turbines comprising the following elements:

-   -   1. A vertically extending pedestal 10 that is cast in situ, on a        barge or dry dock, out of concrete;    -   2. A substantially horizontal support slab 20 that is cast in        situ, on a barge or dry dock, out of concrete, the support slab        20 covering an area of ground larger than that covered by the        pedestal 10;    -   3. A plurality of radial ribs 16 extending radially outwardly        from the pedestal 10 and spaced around the pedestal 10, each rib        being prefabricated and being joined along the base thereof to        the support slab when the support slab 20 is cast in situ and        being joined along an inner side thereof to the pedestal 10 when        the pedestal is cast in situ, each rib has an integral pier for        receiving a leg of lattice lower;    -   4. A plurality of perimeter beams 190 spanning continuously,        near the perimeter of the foundation, between ribs 16 and        supporting the slab 20, optionally each perimeter beam can be        prefabricated;    -   5. A lattice tower 200 having a plurality of legs structurally        connected to the integral piers 180 in the ribs 16, the lattice        tower 200 has, at its top, means for receiving and structurally        supporting a pylon or a tower 300;    -   6. Suitable offshore backfill 13 for weighing down the        foundation, resisting tower loads and providing scour protection        13 b.

When the concrete sets, the pre-cast components will become integralwith a cast-in-place components. Radial post-tensioning tendons extendfrom rib ends opposite rib ends across the pedestal 10. Verticalpost-tensioning is arranged in the pedestal 10 as well. The structuralbehavior is improved by the added compression in all ribs 16, edgebeams, slab 20 and center pedestal.

The lattice tower 200, preferably incorporating 3-dimentional trusses,transfers the pylon loads down to the concrete foundation 100. Thelattice tower 200 may get connected to the concrete foundation prior totransportation or it can be connected to the foundation at finaloffshore installation site.

In another embodiment of the invention, a wind turbine is fabricated ona barge with precast concrete element as following. The barge surface iscoated with a non-bonding agent or covered with a thin membrane at thefoot print where the foundation to be built. Lower slab reinforcing meshsections are assembled and placed and the pedestal cage reinforcing isassembled at the center of the foundation. Upper slab reinforcing meshsections may follow after slab post tension duct is placed. Precastconcrete ribs 16 are placed in a radial array around the pedestal cageand precast concrete perimeter beams 190 are arranged around theperimeter of the foundation. Post tensioning ducts in the pedestal spaceand at perimeter beam-to-rib connections are placed to pair withcorresponding duct in the precast members. Forms for pedestal and forclosure pours at rib-to-perimeter beam connections are installed. Slabconcrete is poured followed by pedestal concrete and closure pours atrib-to-pedestal connections. A lattice tower 200 structure isprefabricated and mounted atop the concrete foundation 100. Thefoundation is transported to installation site using a suitable means oftransportation. Seabed is prepared for receiving the foundation byplacing a sub-base of suitable material such as crushed stone. Thefoundation is backfilled and scour protection measures 13 b areinstalled.

In another embodiment of the invention, the stem 11 is prefabricatedseparately and provided with means for connecting to the pedestal 10,preferably an array vertical post tensioning dowels extended between thepedestal and the stem or other segmental post tensioning joiningmethods. The pedestal may be fitted with means for receiving theprefabricated stem based on segmental post tensioning and groutingconstruction methods.

Piles 400, Micro-piles 401 or piers 402 or suction piers 403 or anchors404 can be used with the offshore foundation 100 in a similar mannerdescribe in the application. In this case vertical sleeves will bearranged in the foundation to receive an array of piles 400 or anchorsextending through the foundation, and allow for additional loadingcapacity and improve stability of foundation. Piles 400 are secured tothe foundation by filling the sleeves with marine grout.

Under some conditions, the use of piles 400, piers or suction piers oranchors may eliminate the slab 20 and/or the perimeter beams 190 fromthe design.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the invention.

1. A foundation comprising: a concrete support slab having horizontalrebar therein, a concrete pedestal integral with the support slab havingvertical rebar therein, a plurality of concrete ribs on top of andintegral with the support slab and integral with the pedestal, the ribsbeing radial to pedestal, and having connection elements extending fromthe ribs into the pedestal and from the ribs into the slab, posttensioning elements extending from the distal end of the ribs throughthe pedestal and, a perimeter beam in the slab having a post tensioningelement therein.
 2. A foundation as in claim 1 wherein, the ribspositioned in pairs on opposite sides of the pedestal and having thetension elements extending from the distal ends of the ribs through thepedestal.
 3. A foundation as in claim 2 wherein, the post tensioningelements in the ribs are eccentrically arrayed for eccentric loading. 4.A fatigue resistant foundation comprising: a horizontally prestressedconcrete slab having a structurally continuous horizontal rebar grid forreinforcing the slab and a plurality of horizontal post tensioningelements therein for prestressing the slab, and a perimeter beamproximate the perimeter of the slab, a vertically extending centralconcrete pedestal having a plurality of vertical post tensioningelements, the pedestal integrally connected to the slab, a plurality ofreinforced concrete ribs on top of and integrally connected to the slabwith structural continuity between the rib and the slab, the ribsintegrally connected to the pedestal, with structural continuity betweenthe rib and the pedestal, at least one post tensioning element runningthrough each rib and the pedestal for providing post tensioning stressin the ribs and pedestal and for connecting the ribs to the pedestalunder post compression stress, such that the pedestal, slab and the ribsare connected to each other to form a monolithic foundation with a threedimensional network of post tensioning elements in the slab, pedestaland ribs keeping the foundation under multi-axial compression whileresisting operating loads, and during normal and abnormal extreme loads,thus linking the slab and the ribs to the pedestal under vertical, andhorizontally eccentric post compression stress, with reduced multi-axialstress range amplitudes in the concrete and steel reinforcing.
 5. Afoundation as in claim 4 wherein, the perimeter beam comprises athickened perimeter slab portion.
 6. A foundation as in claim 5 wherein,the thickened perimeter slab portion comprises a plurality of slabperimeter reinforcing cages, each cage having a plurality of continuoushorizontal bars housed in an array of vertical stirrups extendingbetween the horizontal bars for reinforcing the perimeter slab portionand having a perimeter post tensioning element extending through theplurality of perimeter reinforcing cages to further post tension theperimeter slab portion.
 7. A foundation as in claim 4 wherein, thepedestal has a vertical inner reinforcing cage and an outer reinforcingcage with the plurality of vertical post tensioning elementstherebetween, the pedestal also having a horizontal bottom reinforcingmesh, and a horizontal top reinforcing mesh, with an array of verticalreinforcing hairpin bars extending therebetween for reinforcing the coreof the pedestal.
 8. A foundation as in claim 4 wherein, the pedestal haspairs of ribs on opposing sides of thereof and post tensioning elementsextending from the distal ends of the ribs though the pedestal to holdthe ribs to the pedestal under post compression stress.
 9. A foundationas in claim 7 wherein, the pedestal has a vertical inner reinforcingcage and an outer reinforcing cage with the plurality of vertical posttensioning elements therebetween, the pedestal also having a horizontalbottom reinforcing mesh, and a horizontal top reinforcing mesh, with anarray of vertical reinforcing hairpin bars extending therebetween forreinforcing the core of the pedestal.
 10. A foundation as in claim 9wherein, the pedestal has pairs of ribs on opposing sides of thereof andpost tensioning elements extending from the distal ends of the ribsthough the pedestal to hold the ribs to the pedestal under postcompression stress.
 11. A foundation as in claim 4 wherein, the ribs arecantilevered and have a neutral axis, and the post tension elements inthe perimeter beam and in the ribs and through the pedestal are spacedrelative to the neutral axis of the cantilevered ribs to provide a threedimensional post compression load pattern in the foundation to counterinfluence the multi-stage and multi-axial fatigue loading of the forcesof a wind tower supported thereon when the post tensioning elements aretensioned to the desired stresses.
 12. A foundation as in claim 4wherein, the ribs have a neutral axis and the post tension elements inthe perimeter beam, in the ribs and through the pedestal are spacedrelative to the central axis of the ribs to provide a camber pattern inthe foundation to counter the deformation due to the dead loads on thefoundation from self weight, soil weight and the weight of a wind towersupported thereon when the post tensioning elements are tensioned to thedesired stresses.
 13. A foundation as in claim 4 wherein, the ribs arecantilevered and have a neutral axis such that when perimeter posttensioning elements in the slab are tensioned at an elevation below theneutral axis of the cantilevered ribs it causes an eccentric loading ofthe cantilevered ribs and the pedestal to satisfy the governing loadcase of ribs under downwind loading during extreme normal and abnormalload cases when the post tensioning elements are tensioned to thedesired stresses.
 14. A foundation as in claim 4 wherein, the pedestalis laterally confided by the ribs and the vertical post tensioningelements in the pedestal, and the pedestal further comprises a pluralityof bond-protected anchor bolts extending vertically from an embedmentring near the bottom of the pedestal through the top of the pedestal forsecuring a base flange of a tower in full contact under compression forproviding vertical post compression stress in the foundation when theanchor bolts are tensioned.
 15. A foundation as in claim 4 wherein, abackfilling material placed over the foundation adds stability andstiffness to the foundation.
 16. A foundation as in claim 4 wherein, thesize, distribution, eccentricity and location of post tensioningelements in the ribs and the slab are selected to dictate the naturalfrequencies in the foundation for a desired safety range selected forthe operating frequencies of the wind tower and generator supported onthe pedestal.
 17. A foundation as in claim 4 wherein, the ribs areprefabricated.